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Heat Transfer. How does the energy move from a hotter to a colder object? Three mechanisms Conduction Convection Radiation. Heat Transfer. There are three ways in which heat can be transferred from one object to another: Conduction – when two objects are in physical contact. .
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Heat Transfer How does the energy move from a hotter to a colder object? Three mechanisms Conduction Convection Radiation
Heat Transfer • There are three ways in which heat can be transferred from one object to another: • Conduction – when two objects are in physical contact. k = thermal conductivity Q = heat transferred A = cross sectional area t = duration of heat transfer L = length DT = temperature difference between two ends In a hot oven the air and the metal rack are at the same temperature, but which one feels hotter and why?
Thermal Conductivities, Table 16-3 • Metals have high thermal conductivity, most electrical insulators also have low thermal conductivity. • Air is a great insulator, except that large air spaces allow heat flow by convection.
Convection and Radiation • Convection – when heat is carried by a moving fluid • Example: heat house with radiator • Gulf stream transports Heat from Caribbean to Europe • Radiation – when electromagnetic waves (radiation) carry heat from one object to another. • Example: heat you feel when you are near a fire • Example: Heat from the sun • Formation of frost (ice) at night, T(air) > 0ºC
Conduction Conduction is the process whereby heat is transferred directly through a material, any bulk motion of the material playing no role in the transfer. Those materials that conduct heat well are called thermal conductors, while those that conduct heat poorly are known as thermal insulators. Most metals are excellent thermal conductors, while wood, glass, and most plastics are common thermal insulators. The free electrons in metals are responsible for the excellent thermal conductivity of metals.
Conduction Rate of heat transfer by conduction, Q/t through the length, L across the cross-sectional area, A is given by the following equation, where k is the thermal conductivity and ΔT is the temperature difference between the two ends. SI Unit of Thermal Conductivity: J/(s · m · C°)
Substance Thermal Conductivity, k [J/(s · m · C°)] Metals Aluminum 240 Brass 110 Copper 390 Iron 79 Lead 35 Silver 420 Steel (stainless) 14 Gases Air 0.0256 Hydrogen (H2) 0.180 Nitrogen (N2) 0.0258 High conductivity High conductivity High conductivity
How do fish in a lake survive? Fruit growers sometimes spray water to protect their crops against freezing. After a subzero night, these berries are visible in their insulating jackets of ice.
Convection Convection is the process in which heat is carried from place to place by the bulk movement of a fluid. Convection currents are set up when a pan of water is heated.
Volcanic Eruption During a volcanic eruption, smoke at the top of the plume rises thousands of meters because of convection.
Convection Explains why breezes come from the ocean in the day and from the land at night
Q: In the living room, the heating unit is placed in the floor but the the refrigerator has a top-mounted cooling coil. Why? A: Air warmed by the baseboard heating unit is pushed to the top of the room by the cooler and denser air. Air cooled by the cooling coil sinks to the bottom of the refrigerator.
Radiation Radiation is the process in which energy is transferred by means of electromagnetic waves. Heat transfer by radiation can take place through vacuum. This is because electromagnetic waves are involved in radiation and they can propagate through empty space.
Radiation • Energy carried by electromagnetic waves • Light, microwaves, radio waves, x-rays • Wavelength is related to vibration frequency
Black Body A material that is a good absorber, like lampblack, is also a good emitter, and a material that is a poor absorber, like polished silver, is also a poor emitter.
Suntans Suntans are produced by radiation, ultraviolet rays.
Summer Clothing Q: People are uncomfortable wearing dark clothes during the summer. Why? A: Dark clothes absorb a large fraction of the sun's radiation and then reemit it in all directions. About one-half of the emitted radiation is directed inward toward the body and creates the sensation of warmth. Light-colored clothes, in contrast, are cooler to wear, since they absorb and reemit relatively little of the incident radiation.
A White sifaka Lemur To warm up in the morning, they turn their dark bellies toward the sun.
Why is the mother shielding her cub? Haven’t we seen this before? Ratio of the surface area of a cub to its volume is much larger than for its mother. To cool food, we cut it into smaller pieces, why?
The Stefan–BOLTZMANN Law Of Radiation The rate at which an object emits radiant energy is proportional to the fourth power of its absolute temperature. This is known as Stefan’s law and is expressed as follows, where σ is the Stefan-Boltzmann constant, σ = 5.67 10-8 W/m2.K4. The factor e is called the emissivity, which is a number between 0 and 1. Perfect radiators have a value of 1 for e. A is the surface area and T is the temperature of the radiator in Kelvin.
Thermos Bottle A thermos bottle minimizes energy transfer due to convection, conduction, and radiation. Stopper- minimize conduction. Double-walled glass vessel with the space between the walls is evacuated to minimize energy losses due to conduction and convection. The silvered surfaces reflect most of the radiant energy that would otherwise enter or leave the liquid in the thermos.
Halogen Cooktop In a halogen cooktop, quartz-iodine lamps emit a large amount of electromagnetic energy that is absorbed directly by a pot or pan.
Metal foil Highly reflective metal foil covering this satellite minimizes heat transfer by radiation.
Conduction • There are three ways in which heat can be transferred from one object to another: • Conduction – when two objects are in physical contact. k = thermal conductivity Q = heat transferred A = cross sectional area t = duration of heat transfer L = length DT = temperature difference between two ends In a hot oven the air and the metal rack are at the same temperature, but which one feels hotter and why?
Thermal Conductivities, Table 16-3 • Metals have high thermal conductivity, most electrical insulators also have low thermal conductivity. • Air is a great insulator, except that large air spaces allow heat flow by convection.
Convection and Radiation • Convection – when heat is carried by a moving fluid • Example: heat house with radiator • Gulf stream transports Heat from Caribbean to Europe • Radiation – when electromagnetic waves (radiation) carry heat from one object to another. • Example: heat you feel when you are near a fire • Example: Heat from the sun • Formation of frost (ice) at night, T(air) > 0ºC
Conduction Conduction is the process whereby heat is transferred directly through a material, any bulk motion of the material playing no role in the transfer. Those materials that conduct heat well are called thermal conductors, while those that conduct heat poorly are known as thermal insulators. Most metals are excellent thermal conductors, while wood, glass, and most plastics are common thermal insulators. The free electrons in metals are responsible for the excellent thermal conductivity of metals.
In contrast to conductors, insulators are materials which impede the free flow of electrons from atom to atom and molecule to molecule. If charge is transferred to an insulator at a given location, the excess charge will remain at the initial location of charging.
In a conductor, electric current can flow freely, in an insulator it cannot. Metals such as copper typify conductors, while most non-metallic solids are said to be good insulators, having extremely high resistance to the flow of charge through them. "Conductor" implies that the outer electrons of the atoms are loosely bound and free to move through the material. Most atoms hold on to their electrons tightly and are insulators. In copper, the valence electrons are essentially free and strongly repel each other. Any external influence which moves one of them will cause a repulsion of other electrons which propagates, "domino fashion" through the conductor. Simply stated, most metals are good electrical conductors, most nonmetals are not. Metals are also generally good heat conductors while nonmetals are not.
This website has a very good introduction. However, it is in form of a flash movie, so I cannot put it on a PowerPoint presentation. I believe it can be done, but I do not know how. http://www.thetech.org/exhibits/online/topics/10a.html
Black Body Radiation • Any object heated to a temperature T (on an absolute scale) radiates Electromagnetic Energy (light) with total power: • P = e s A T4 • 0<e<1 = emissivity = property of material • s = 5.67 ·10 –8 W/(m2 K4) • A = surface area of object • Early triumph of quantum theory (M. Planck) to predict this equation, including the value of s. • Peak wavelength occurs at l = (5.1·10-3 m ·K ) / T (Chap 30) • If the surroundings have temperature TS, then the net power radiated is • P = e s A [ T4 - TS4] • Dark, dry, night, TS = 3 K, Black body radiation cools the surface faster than conduction can transport heat from the ground or air. Frost can form even if air temperature > 0C.
